Transmitter

ABSTRACT

A transmitter for providing inventory or inventory transfer information on industrial sites is described, for measurement of mass (M), weight (W), volume (V), level (L) and/or density (ρ) of a product ( 1 ) in a vessel ( 3 ) comprising: a level sensor ( 5 ), comprising: a conductive probe ( 9 ) extending into the vessel ( 3 ), means ( 11, 13 ) for generating and sending short electromagnetic pulses (S) down the probe, means ( 15 ) for reception of echoes (E) of the pulses (S) reflected at a surface of the product ( 1 ), means ( 17 ) for determining a time of flight (t) needed for a pulse (S) to travel down the probe ( 9 ) and its echo (E) to return, a pressure sensor ( 7 ), mounted on the probe ( 9 ), and a signal processing unit ( 45 ) for determining mass (M), weight (W), volume (V), level (L) and/or density (ρ) based on measurement signals supplied by the level sensor ( 5 ) and the pressure sensor ( 7 ).

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a transmitter.

In many branches of industry, products are stored in vessels, forexample in tanks or containers. Suppliers supply the product to thevessel and users obtain it from the vessel. Since industrial productscan be very expensive complex inventory systems are used to account forin and out going product.

A product received from a supplier may be valued in volume (i.e. litersor gallons) when delivered and valued as a weight (i.e. kilograms orpounds) as plant inventory. The reconciliation of incoming product valueand storage value can result in financial losses if not properlymanaged. As bulk supply chains become more collaborative, product withina given vessel may have more than one owner. Tracking product inventoryaccurately becomes more important. Suppliers and users do not alwayshave the appropriate volumetric or mass flow measurement devices on siteto account for usage. If platform scales are not available at thedelivery site to weigh an incoming bulk liquid delivery truck, valuationof the delivered bulk liquids given by the supplier is assumed to becorrect by the user. If a custody transfer station is not available atthe delivery site, a valuation of the delivered bulk liquids given bythe supplier is assumed to be correct by the user.

At present individual measurement devices are used to get information oninventory of products in a tank.

Pressure measurement devices mounted on a sidewall of the vessel or on acable or rod extending into the vessel determine hydrostatic pressureexerted on the device by the product above it. Algorithms are used toconvert this hydrostatic pressure into level or volume values. Level orvolume values are not fully corrected for density or temperaturechanges. Temperature measurement devices mounted on a sidewall of thevessel or inside the vessel on a cable or rod are used to determine thetemperature of the liquid. Algorithms are used to correct the level orvolume information for temperature variation. This requires anotheropening in the tank to accomplish.

Devices mounted on a sidewall of the vessel can be subject to widertemperature differences between product and ambient temperatures. Inaddition externally mounted pressure sensors are subject to tank wallmovements, when the tank bulges out when filled, that can introducesignificant errors.

A position of a surface of the product within the vessel can bedetermined with top mounted electronic measuring devices usingcapacitance, ultrasonic, radar, laser technologies. Algorithms are usedto convert the position of the liquid into inventory level or volumeinformation. Again, these values are not fully corrected for density ortemperature changes. Often an additional pressure transmitter is used tocalculate mass or density corrected volume. These hybrid solutionsrequire multiple tank openings to accomplish.

Determination of the mass or weight of a liquid in a pressurized tankmay require the addition of another pressure sensing device to accountfor a head pressure inside the tank above the product.

Determination of the density of the product is required by somealgorithms to calculate corrected volume or level. In order to determinethe density an additional pressure sensing device is mounted some knowndistance above the hydrostatic pressure measurement device. Density isthen determined based on the distance between the two devices and thedifference in pressure at their respective locations.

Another method to determine product inventory within a tank is to usemechanical floats or displacers mounted on a cable or rod inside thetank. The position of these floats is used to determine the level of theliquid. Magnetostrictive, resistance or cable tension sensingtechnologies are used to determine the position of the float. Anadditional displacer may be used to detect the density of the liquid atsome point below the level measurement. Mechanical floats and displacersoften require significant maintenance and are subject to materialbuildup errors. They often require physical entry into a tank to repair.

Load cell and strain gauge sensors are used to determine the weight ofproduct in a tank. It is not always possible to install such sensors dueto technical limitations and high installation cost.

More recently time of flight radar techniques have been used todetermine the position of the surface of a product in a tank.Corresponding level measurement devices comprise a conductive probe thatextends into the vessel. Microwave energy pulses are transmitted via acoupling, for example a launch plate down the probe. Some energy isreflected by the surface and returned back up the probe. A transit timefrom launch pulse launch to return can be used to calculate the positionof the product surface.

Current practice is to use current loops to convey each individualmeasurement value from each measurement device to a programmable logicalcontroller (PLC), a distributed control system (DCS) or othercalculation device to perform the algorithms required to scale,compensate and correct the inventory information. Each measurementtransmitter, its installation and wiring, adds cost.

If wireless technology is employed continuous power consumption fromeach transmitter can be over 0.5 watts.

Often process control measurement transmitters are employed in inventorymeasurement applications. The update times for each transmitter oftenexceed one second increasing power consumption for each device used.Inventory applications generally need to provide periodic, timed orevent based information.

In the petroleum industry additional requirements need to be fulfilled.The American Petroleum Institute (API) defines the practice of measuringpetroleum products in terms of corrected volume. The liquid position andliquid temperature at one or more points is used to calculate theinventory volume at a reference temperature using well known algorithms.Direct mass or weight measurement approaches are not generally used.There is increasing interest in using mass and/or weight determinedvalues of petroleum products however, the current practices do notbroadly support development of this information.

Floats and displacer gauging systems use mechanically moving componentsand sensors to measure level, temperature and density of products.

Radar gauges often combined with externally mounted temperature sensorsare used to determine corrected level and volume.

Hydrostatic tank gauging systems use up to three externally mountedpressure transmitters, a temperature transmitter and a calculation boxto calculate mass and sometimes density.

Hybrid systems use a top mounted radar gauge to determine level andexternally mounted temperature transmitters and sometimes externallymounted pressure transmitters. A calculation box is installed tocalculate mass, corrected volume and sometimes density.

Various digital buses generally of a proprietary design are used toconvey the level, temperature and sometimes pressure information to thecalculation box for volume correction and then to an inventory reportingand/or management system. Various types of converter boxes are used totransform one digital protocol into another at some significant expensewhen replacement of measurement technologies and/or suppliers areintegrated into existing installations.

It is an object of the invention to provide a transmitter for providinginventory or inventory transfer information on industrial sites.

To this end the invention comprises a sensor apparatus for measurementof mass, weight, volume, level and/or density of a product in a vesselcomprising:

-   -   a level sensor, comprising:        -   a conductive probe extending into the vessel,        -   means for generating and sending short electromagnetic            pulses down the probe,        -   means for reception of echoes of the pulses reflected at a            surface of the product,        -   means for determining a time of flight needed for a pulse to            travel down the probe and its echo to return,    -   a pressure sensor,        -   mounted on the probe, and    -   a signal processing unit for determining mass, weight, volume,        level and/or density based on measurement signals supplied by        the level sensor and the pressure sensor.

According to a preferred embodiment, the transmitter comprises at leastone temperature sensor, integrated in the probe.

According to a preferred embodiment, the transmitter comprises acommunication interface for reception and/or delivery of information toa user, a supplier and/or a control unit.

According to a preferred embodiment, the transmitter comprises atotalizer, for totalizing supplies or withdrawals of the product.

According to a preferred embodiment, the transmitter comprises amonitor, for monitoring unauthorized supply or withdrawal of product andleakage.

According to a preferred embodiment, the transmitter comprises a devicefor determining a physical position of the transmitter.

According to a preferred embodiment, the transmitter comprises anintegral server for supporting communication with at least oneInformation Technology Network.

The transmitter according to the invention provides multiple inventoryinformation variables from one measurement device. Compensation andcorrection of any of these output values can be performed by thetransmitter itsself. The need for external programmable logicalcontrollers (PLC), distributed control systems (DCS) or othercalculation boxes to provide correct inventory information iseliminated. Pressure and temperature sensors are incorporated on thesame probe used for time domain reflectometry.

The sensors are mounted internal to the vessel so they will generally beat the same temperature as the product.

The transmitter is able to provide level, weight, volume and/or densityinventory information over time to inventory logistics operations thatuse this information to determine if a product delivery is required orthat there is room to receive a delivery. The transmitter cansimultaneously provide the information to both suppliers and users.

The invention and its advantages are explained in more detail using thefigures of the drawing, in which one exemplary embodiment is shown. Thesame reference numerals refer to the same elements throughout thefigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a transmitter according to the invention; and

FIG. 2 shows the probe of the transmitter in FIG. 1, a pressurecompensation port and temperature sensors.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a transmitter according to the invention for measurement ofmass, weight, volume, level and/or density of a product 1 in a vessel 3.The transmitter comprises a level sensor 5 and a pressure sensor 7.

The level sensor 5 is mounted on an opening located on top of the vessel3. It comprises a conductive probe 9 that extends into the vessel 3. Theprobe 9 can for example be a rod or a cable extending into the vessel 3.

The level sensor 5 comprises means 11, 13 for generating and sendingshort electromagnetic pulses S down the probe 9, means for reception 15of an echo E of the pulse reflected at a surface of the product 1 andmeans 17 for determining a time of flight needed for a pulse S to traveldown the probe 9 and its echo E to return.

The means 11, 13 for generating and sending short electromagnetic pulsesdown the probe 9 comprise an internal clock 19 and a pulse generator 21.The internal clock 19 generates a pulse emission rate and supplies it tothe pulse generator 21. Recent developments by the National LaboratorySystem now make it possible to generate fast, low power pulses, and timetheir return with very inexpensive circuits. See, for example, U.S. Pat.No. 5,345,471 and U.S. Pat. No. 5,361,070 assigned to The Regent of theUniversity of California. The pulses generated by this new technologyare broadband, and are not square wave pulses. In addition, thegenerated pulses have a very low power level. Such pulses are at afrequency of 100 MHz or higher and have an average power level of about1 nano Watt or lower. Typical emission rates are for example severalMHz.

The pulses S are supplied to the probe 9 via a coupling 23. Theypropagate down the probe 9, are reflected at the product surface andreturn to the coupling 23. From there they are supplied to the means 15for reception of the echo E comprising a directional coupler 25 and ahigh pass-filter 27.

The filtered signal is supplied to the means 17 for determining the timeof flight of the pulses.

These means 17 comprise a delay line 29. The delay line is connected tothe clock 19 and produces a sampling rate that is equal to the pulseemission rate produced by the clock 19 delayed by a variable delay time.

The variable delay time varies for example according to a saw toothfunction. The sampling rate is supplied to a sampling pulse generator31, which in turn generates sampling pulses and provides them to a firstinput of a sample and hold circuit 33. Preferably sampling pulsegenerator 31 and pulse generator 21 are identical.

The filtered signals obtained by the means 15 for reception are suppliedto a second input of the sample and hold circuit 33.

In operation short pulses S are sent periodically according to the pulseemission rate and their echo signals are supplied to the sample and holdcircuit 33. The sample and hold circuit 33 superimposes a sampling pulseon every echo signal and the resulting signal is supplied to anamplifier 35. The amplified signal is digitalized by an analog todigital converter 37 connected in series to the amplifier 35. A digitaloutput of the analog to digital converter 37 is supplied to amicroprocessor 39.

The resulting signal is a measure for a correlation between the echosignal and the sampling pulse. Echo signals arrive periodically. Theydiffer from the sampling pulses by the delay time, which increasesaccording to the saw tooth function.

Assuming that consecutive echo signals are nearly identical, the sampleand hold circuit 33 provides a stroboscopic recording of the echosignals. This assumption is generally correct, because the level insidethe vessel essentially does not change between to consecutive pulses.

The microprocessor 39 is linked to the clock 19 and the sampling rategenerator 29.

The microprocessor 39 regularly starts measurement cycles. During eachmeasurement cycle, pulses S are generated and send periodically andtheir echoes E are received and corresponding echo signals are sampledas described above. A measurement cycle ends, when all delay timesaccording to the saw tooth function were applied. The sampling valuesprovided by the analog to digital converter 37 and the correspondingdelay times are recorded. The sampling values as a function of the delaytime show a distinct maximum at a delay time that is equal to the timeof flight needed for the pulse to travel down the probe 9 and for itsecho signal to return. This time of flight t is determined by themicroprocessor 39.

The time of flight thus determined is proportional to the distance Dbetween the coupling 23 and the surface of the product 1. Given avelocity v of propagation of the pulse along the probe and a distance Hbetween the coupling 23 and a bottom of the vessel 3, the level Lequals:L=H−½(vt).The pressure sensor 7 is mounted on the probe 9. Preferably, it ismounted on or near an end of the probe 9 near the bottom of the vessel3. Sensor wiring can be run externally alongside the probe 9,incorporated under a protective shield surrounding the probe 9 orintegrated inside the probe 9. The pressure sensor 7, for example acompact hydrostatic pressure sensor, produces an output proportional toa pressure p at its position near the bottom of the vessel 3.Hydrostatic pressure sensors generally consist of a membrane, which ismechanically or hydraulically connected to a transducer element, whichcan be based on inductive, capacitive, strain gauge or semiconductorprinciples. The pressure p is produced by a column of liquid of a heightHL above the pressure sensor 7.

The pressure p depends on the height HL according to the followingformula:p=ρgHL

-   -   wherein ρis an average density of the product    -    g is an acceleration due to gravity, and    -    HL is the height of the column.

Preferably, a signal-pre-processing and amplification unit isincorporated inside the pressure sensor 7 inside the vessel 3. Itsoutput is provided to an electronic circuitry 41 for further processing.

In vessels 3 open to atmosphere, the surface of the product 1 is exposedto atmospheric pressure. Most pressure sensors 7 compensate foratmospheric pressure, such that the pressure p measured is equal to thedifference between an absolute pressure at the location of the pressuresensor 7 and atmospheric pressure. For compensation purposes, a pressurecompensation port 43 can be foreseen. The pressure compensation port 43is connected to a sensing element of the pressure sensor 7. It islocated above the level of the product 1. Preferably, it is integratedin the probe 9 near a top of the vessel 3.

If the pressure inside the vessel 3 above the product 1 is greater thanatmospheric pressure, an additional pressure sensor needs to beinstalled to measure a head pressure acting on the surface. In thiscase, the pressure acting on the surface is subtracted from the totalpressure, leaving only the pressure p due to the column of the liquid inthe vessel 3 above the pressure sensor 7.

The transmitter comprises a signal-processing unit 45 for determiningmass M, weight W, volume V, level L and/or density ρ based onmeasurement signals supplied by the level sensor 5 and the pressuresensor 7. The supplied measurement signals are the pressure p and thetime of flight t.

Given the pressure p and the time of flight t, mass M, weight W, volumeV, level L and density ρ are calculated according to the followingformulas:

Level L:L=H−½(vt) wherein

-   -   H is the distance between the coupling 23 and a bottom of the        vessel 3,    -   v is the velocity of propagation of the pulse, and    -   t is the time of flight of the pulse.        Average density ρ:        ρ=p/(gHL)=p/(g(L−Hd)) wherein    -   p is the pressure,    -   g is the acceleration due to gravity,    -   L is the level, and    -   Hd is a distance Hd between the pressure sensor 7 and the bottom        of the vessel 3.        Mass M:        M=ρAL=ρAL/(g(L−Hd)) wherein    -   p is the pressure of the column,    -   A is a crossectional area of vessel 3,    -   L is the level,    -   g is the acceleration due to gravity,    -   L is the level, and    -   Hd is a distance Hd between the pressure sensor 7 and the bottom        of the vessel 3.        Weight W:        W=gM=ρAL/(L−Hd) wherein    -   p is the pressure of the column,    -   A is a crossectional area of vessel 3,    -   L is the level,    -   g is the acceleration due to gravity,    -   L is the level, and    -   Hd is a distance Hd between the pressure sensor 7 and the bottom        of the vessel 3.        Volume V:        V=AL wherein    -   A is a crossectional area of vessel 3, and    -   L is the level.

The signal-processing unit 45 is connected to the microprocessor 39 ofthe level sensor 5 and to the electronic circuitry 41 of the pressuresensor 7. It comprises one or more memories 47 for storing data relatedto the vessel 3 and/or the transmitter, software and/or measurementdata.

In the embodiment shown, the pressure p and the time of flight t aresupplied to the signal-processing unit 45 and it calculates mass M,weight W, volume V, level L and/or density ρ based on the pressure p,the time of flight t and the information stored in the memories.Alternatively, some of these calculations can be performed by themicroprocessor 39 or the electronic circuitry 41.

In order to allow for temperature compensation, the sensor apparatus cancomprise at least one temperature sensor 49. The temperature sensors 49are integrated in the probe 9, as shown in FIG. 2, and supplytemperature information to the signal processing unit 43. Thetemperature information can be supplied to the means 17 for determiningthe time of flight and to the electronic circuitry 41 to allowtemperature compensation of the pressure p and the time of flight t tobe measured. Alternatively, temperature compensation of the pressure pand the time of flight t, as well as temperature compensation of mass M,weight W, volume V, level L and density ρ can be performed by thesignal-processing unit 45.

Preferably, the transmitter comprises a communication interface 51 forreception and/or delivery of information to or from a user, a supplierand/or a control unit 52.

In addition, the transmitter comprises a totalizer 53, for totalizingsupplies and/or withdrawals of the product.

In the embodiment shown, the totalizer 53 is part of thesignal-processing unit 45. The totalizer 53 tracks and stores everysupply and withdrawal of product. The information gathered and stored bythe totalizer is accessible via the communication interface 51. Thisallows inventory information to be obtained at any time. The transmitterprovides this information in terms of mass M and/or volume V. Suppliersand users are therefore free to use whichever physical unit they prefer.

Supply and/or withdrawal information can be provided to billing andconsignment operations to determine product usage out of the vessel 3and product introduction into the vessel 3. This can for example be usedto monitor mass transfer in a standalone tank with appropriate fill andwithdrawal procedures. Reconciliation algorithms can be employed toreconcile the movement of incoming and outgoing product.

Information from the totalizer 53 can be combined with informationreceived from or delivered to sources outside via the communicationsinterface 51. The accuracy can be sufficient to use in place of inputand output flow meters especially if part of a consigned bill-when-usedcontract program and/or when it is difficult or expensive to properlyinstall and operate pipe-mounted flow meters.

Further, the transmitter comprises a monitor 55, for monitoringunauthorized supply or withdrawal of product and leakage. In theembodiment shown, the monitor 55 is part of the signal-processing unit45. The monitor 55 surveys the inventory information and compares itwith information on authorized supplies and/or withdrawals, which can besupplied via the communication interface 51.

Monitoring can for example be performed by delivering mass informationto statistical reconciliation algorithms. Preferably, this is doneduring quiescent periods. Whenever the product content of the vessels 3changes without any authorization information being supplied to themonitor 55, the monitor 55 will issue a warning or an alarm.

The transmitter can be equipped with open fieldbus communication means,such as bus powered HART, Profibus, Foundation Fieldbus or Power overEthernet (PoE) TCP/IP to report information and support remote servicingand asset management. Alternatively, means for wireless communicationcan be foreseen. Hard wiring of equipment is one of the significantcosts of inventory measurement instrumentation. Instead of multiplelevel, pressure, temperature devices mounted individually on each vesseleach with its own transceiver, the transmitter according to theinvention requires only one opening and one transceiver. This reduceswiring and mounting costs.

In addition, the transmitter requires only one single power supply. Thepower needed by the level sensor 5, the pressure sensor 7 and thetemperature sensors 49 to produce the information required isdistributed within the transmitter. This allows to optimize power usagewhen required. Each sensor can be powered as needed to provide requiredinformation and optimize power usage. For example, during fillactivities sample times can be increased when inventory changes aregreater than a defined rate. During quiescent times sample times andtimes during which sensors are powered can be reduced. Inventoryreporting can be on a periodic and/or exception basis. Battery, solararray, or fuel cell sources can be used making self powered wirelessoperation feasible.

In application, where the transmitter shall be mounted on a mobilevessel 3, or can be located in various different areas of an industrialsite, the tranmitter is preferably equipped with a device 54 fordetermining a physical position of the transmitter, for example a globalpositioning system (GPS). The device 54 can for example be linked to thesignal processing unit 45, so that information about the physicalposition of the transmitter is accessable together with the measurementdata.

Existing proprietary buses used within the petroleum industry such asMarkspace, TIWAY and others can be used within the invention toeliminate external calculation devices.

In addition an integral server 56 can be provided within thetransmitter, for supporting communication with at least one InformationTechnology Network 58.

The use of self-powered wireless network technologies allows one to addwireless multivariable measurement versions of the transmitter apparatusaccording to the invention one at a time to a tank farm parallel to anexisting legacy proprietary bus. The legacy bus can be phased out whenits installed base is displaced. Fuel cell, battery or solar power andTCP/IP wireless or other LAN or WAN wireless technologies can be used.

The transmitter provides integrally mounted sensing elements that can beinstalled and removed from the vessel 3 with out affecting the sides orbottom of the vessel 3. The individual pressure sensor 7, level sensor 5and temperature sensors 49 are combined into one measurement device.Installation is similar to traditional top mounted mechanical/electronicinstruments. Measurement devices mounted on the side or bottom of a tankare eliminated. The inventory measurement openings in the vessel 3 arereduced to one helping to eliminate potential emissions or leakagesources.

1. A transmitter for measurement of mass (M), weight (W), volume (V),level (L) and/or density (ρ) of a product (1) in a vessel (3)comprising: a level sensor (5), comprising: a conductive probe (9)extending into the vessel (3), means (11, 13) for generating and sendingshort electromagnetic pulses (S) down the probe, means (15) forreception of echoes (E) of the pulses (S) reflected at a surface of theproduct (1), means (17) for determining a time of flight (t) needed fora pulse (S) to travel down the probe (9) and its echo (E) to return, apressure sensor (7), mounted on the probe (9), and a signal processingunit (45) for determining mass (M), weight (W), volume (V), level (L)and/or density (ρ) based on measurement signals supplied by the levelsensor (5) and the pressure sensor (7).
 2. A transmitter according toclaim 1, comprising at least one temperature sensor (49), integrated inthe probe (9).
 3. A transmiter according to claim 1, comprising acommunication interface (51) for reception and/or delivery ofinformation to a user, a supplier and/or a control unit (52).
 4. Atransmitter according to claim 1, comprising a totalizer (53), fortotalizing supplies or withdrawals of the product (1).
 5. A transmitteraccording to claim 1, comprising a monitor (55), for monitoringunauthorized supply of withdrawal of product (1) and leakage.
 6. Atransmitter according to claim 1, comprising a device (54) fordetermining a physical position of the transmitter.
 7. A transmitteraccording to claim 1, comprising an integral server (56) for supportingcommunication with at least one Information Technology Network (58).